USB technology allows many peripherals to be connected using a single standardized interface socket and improves the plug-and-play capabilities by allowing hot swapping. Hot swapping allows devices to be connected and disconnected without rebooting the computer or turning off the device. USB technology can connect computer peripherals such as a mouse, keyboards, PDAs, gamepads and joysticks, scanners, digital cameras, printers, personal media players, flash drives, and other devices.
A traditional USB cable has four wires and connections. The outside two conductors, VBUS and GND, provide power for the USB device, if needed. The center two conductors, D+ and D−, are the differential data pair which uses half-duplex differential signaling to communicate data between the USB device and a host. Since the differential data pair conductors D+ and D− are physically near the power and ground conductors, an electrical transient from a host power supply can cause noise on the differential data pair conductors D+ and D− and, hence, disrupt data communications between the host and the USB device.
The International Electrotechnical Commission has published IEC 61000-4-4, which is a standard for measuring and testing an electrical fast transient/burst (EFT/B) which can occur on a USB data cable. This standard establishes a common and reproducible reference for evaluating the immunity of electrical and electronic equipment when subjected to EFT/Bs. EFT/B tests are carried out up to +/−2.0 KV or higher with durations of up to 60 seconds. At these voltages and with these time periods, the data transfer between the host and the USB device is corrupted. When this occurs, the host sends a reset command to the USB device, and the USB device attempts to send a series of replies as outlined in the USB specification. However, if the replies from the USB device during the reset sequence are also corrupted by the EFT/B event, then the host will not recognize the replies and eventually stops trying to reset the connection with the USB device. When the host stops trying to reestablish communications with the USB device, the host places the USB port into a suspend mode or state. When a USB port has been placed into a suspend mode, the USB device is manually disconnected and reconnected to the host, which can be irritating and time consuming.
FIG. 1 illustrates a graphical waveform diagram 10 of a data signal 14 affected by an EFT/B sequence 12 on a data bus of a conventional USB device. The individual bursts (designated as B1 through B5) are shown having a typical duration of Tbd. This duration could be 15 ms in duration, for example. The time period of the bursts is shown as Tb. The time Tb could be 300 ms, for example. The times Tbd and Tb could vary, but are shown for illustration purposes.
The exchange of data between the USB device and the host becomes corrupted with each EFT/B event because there is an electric field emitted from the VBUS and GND conductors in the USB cable during each EFT/B strike on the host's power supply. The electric field generated from the EFT/B strike causes the data bus to toggle during the edges of the burst, which disrupts the exchange of data and/or causes data bus states not allowed in the USB specification. These irregularities prompt the host to reset the communication with the USB device. As the reset transaction can be unsuccessful due to the bursts, the host will try unsuccessfully and then put the USB port into suspend mode. Time 0 in FIG. 1 is a time at which the data communications between the USB device and the host are normal until the first EFT/B event B1. The period between the first EFT/B event B1 and Time 1 is the period of time when the host attempts to reset the USB port communications. If unsuccessful, then at Time 1 the host places the USB port and device into suspend mode. Time 2 is a point in time after the EFT/B events have stopped. Although the EFT/B events have stopped by Time 2, the USB port remains in the suspend state until a user manually disconnects and reconnects the USB device to the host.
Traditional methods to improve immunity of USB devices to EFT/B events employ conventional passive implementations. One example of a conventional passive implementation is power filtering using bypass capacitors across the VBUS and GND connections. Another example of a conventional passive implementation is utilizing a high quality USB cable with ferrite core and good shielding characteristics. Another example of a conventional passive implementation is proper shielding of the USB data bus (D+, D−) on the PCB on the USB device. All of these conventional methods result in higher system cost and bigger device size. For protection against extremely strong EFT/B events, these conventional passive methods may not be able to provide sufficient protection, and the host could nevertheless place the USB device into a suspend mode even with passive EFT/B protection.